1. Field of the Invention
This invention relates to precipitation polymerization of copolymers of a vinyl lactam and a polymerizable carboxylic acid, and, more particularly, to such copolymers having a wide compositional range, prepared in high yield, as a white powder, which can be filtered and dried easily, and which have an advantageous low molecular weight of less than 20,000.
2. Description of the Prior Art
Copolymers of vinyl lactams, e.g. vinyl pyrrolidone (VP) or vinyl caprolactam (VCL), and polymerizable carboxylic acids, e.g. acrylic acid (AA) or methacrylic acid (MAA), have found application in the photographic industry, as coatings, as biological membranes, in drug release systems, as preservatives, in oil recovery processes, in immunochemicals, and in cosmetic materials.
Solution polymerization and precipitation polymerization are two available methods for making these copolymers; see U.S. Pat. Nos. 3,044,873; 3,862,915; and 4,283,384; articles by Takeski, in J. Makromol. Chem. 148, 205 (1971), and by Uelzmann, in J. Polymer Sci. 33, 377 (1958); and Belgian Patent No. 668,368.
The solution polymerization process is used when both reactant monomers and the copolymer product are soluble in a reaction solvent. However, this method suffers from the following disadvantages:
(1) Desirable compositions of the copolymer may not be soluble in a selected solvent; (2) the yields of such copolymer may be low; (3) the copolymer may be colored; (4) the solvent may be a high boiling liquid which is difficult to separate from the copolymer; and (5) the solvent may be a protic liquid, e.g. water or mixtures thereof, which causes considerable hydrolysis of the vinyl lactam under acidic reaction conditions.
The precipitation polymerization method is useful when the monomers are soluble in the reaction solvent and the copolymers are insoluble in the solvent. Benzene, tetrahydrofuran, heptane, acetone and methyl ethyl ketone are known precipitation polymerization solvents. Unfortunately, these solvents have one or more of the following disadvantages:
(1) Useful copolymer compositions may not be insoluble in a selected reaction solvent, which restricts the process to a narrow copolymer compositional range; (2) the copolymer may precipitate only as a gelatinous mass which is difficult to filter; (3) low yields of polymer may be obtained; (4) the solvent may be toxic; and (5) low molecular weight copolymers may be difficult to make by available processes.
For these and other reasons, present solution polymerization and precipitation polymerization processes for making low molecular weight copolymers of a vinyl lactam and an acrylic acid have not been very successful on a commercial scale.
Accordingly, it is an object of this invention to provide an improved method of making low molecular weight copolymers of a vinyl lactam and a polymerizable carboxylic acid, particularly copolymers of vinyl pyrrolidone and acrylic acid having an advantageous low molecular weight of less than 20,000.
Another object of the present invention is to provide an effective reaction solvent in a precipitation polymerization process which will provide copolymers having advantageous physical properties, including low hygroscopicity, high glass transition temperatures, and, particularly, low average molecular weights.
Yet another object herein is to provide a precipitation polymerization process in which the copolymers will precipitate in high yield as a fine white powder which is insoluble in the reaction solvent over the entire compositional range of the copolymer.
A feature of the invention is the provision of a precipitation polymerization process for making copolymers of a vinyl lactam and a polymerizable carboxylic acid in which a mixture of an aliphatic hydrocarbon and a secondary alcohol such as isopropanol is used as the reaction solvent.
Another feature of the invention is the provision of copolymers of a vinyl lactam and a polymerizable carboxylic acid made by a precipitation polymerization process in which the reaction solvent is a mixture of heptane and a secondary alcohol such as isopropanol, the latter being present in an amount of about 30% or less, preferably 5-15%, by weight of the mixture.
Still another feature of the invention is the provision of copolymers of a vinyl lactam and a polymerizable carboxylic acid over the complete compositional range of monomers, which copolymers are characterized by having a low weight average molecular weight, Mw, of less than about 20,000, low hygroscopicity, a high glass transition temperature, being a white powder, exhibiting polyelectrolyte behavior, and which are soluble in an aqueous basic solution of pH 8 and insoluble in an aqueous acid solution of pH 3.